A device on a double thread lockstitch sewing machine for recognizing a residual amount of the hook thread has been known from DE 41 16 638 A1, in which the reaching of the beginning of the residual amount during the pull-off of the hook thread, which takes place during sewing, is recognized from the speed of rotation of the bobbin, which progressively increases with decreasing amount of hook thread, exceeding a predetermined threshold value compared with the speed of rotation of the main shaft of the sewing machine.
Since the rotary movement of the bobbin during sewing is a movement composed of partial movements due to the law governing double lockstitch formation and the stepwise transport of the fabric being sewn, and the bobbin is subject in this connection, especially as a consequence of the jerky pulling in of the stitch by the thread lever, to pulse-like accelerations whose effect on the rotation behavior of the bobbin depends, besides on the stitch length, also on the value and possibly the changes in the speed of rotation of the machine as well as on the coefficient of friction between the bobbin and the bobbin case of the hook, a standardized mean speed of rotation, which would be able to be calculated as a theoretical value from the current speed of rotation of the machine, a defined stitch length and the thickness of the fabric, as well as from the current diameter of the hook thread winding on the bobbin, cannot be inferred from a measured instantaneous speed of rotation of the bobbin. Since the instantaneous speed of rotation of the bobbin oscillates to various extents around the value of the standardized mean speed of rotation, which increases continuously with decreasing amount of hook thread, considerable efforts were made in the prior-art device in terms of signal processing to overcome these inaccuracies. Thus, a mean value is first formed from all speed measured values after the conclusion of a sewing cycle, and this mean value is then compared with the mean value of the preceding sewing cycle to determine a coefficient of change. This calculated value is then compared with a threshold value, which represents the value of the residual amount of thread and is in turn calculated by the computer of the sewing machine on the basis of values entered manually for the stitch length and for the fabric thickness.
A comparatively much more accurate and generally simpler determination of the beginning of a residual amount of hook thread is achieved in the device according to DE 41 16 788 C1 by the amount of residual thread being wound in a desired length in a winding direction that is opposite the winding direction of the amount of the main thread. The direction of rotation of the bobbin changes at the time of the transition from the principle amount to the residual amount of thread, and this change can be unambiguously recognized by measurement and can be subjected to further processing as a signal in a simple manner. However, this advantage is associated with the disadvantage that the usual winding devices, which are sometimes also arranged on the machines, cannot be used to wind thread on the bobbins, bur special left-right bobbin winders are needed.
A device for recognizing a residual amount of hook thread has been known from DE 40 31 058 C1, This device has a bobbin which has, due to the arrangement of at least one third bobbin flange, at least two chambers, which are separated from one another and one of which is used to accommodate the residual amount of thread. The middle flange has a reflecting surface at least on the side facing a reflection head containing a light transmitter and a light receiver. During the winding up of thread, the desired residual amount of thread is first wound up in the rear chamber on the side of the middle flange facing away from the reflecting head, after which the thread is led past the middle flange and the front chamber is then filled.
During sewing, the thread is first pulled out of the front chamber near the end of the unwinding process before the residual amount of thread is pulled off, as a result of which the light beams emitted by the reflecting head are reflected from the middle flange that has become free. The signal generated by the reflected light beams is compared with an adjustable threshold value, and a warning signal is generated when this is exceeded. Depending on the setting of the threshold value, a more or less large residual thread length may now still be present in the front chamber, which is added to the actual residual amount of thread contained m the rear chamber. Since the intensity of the reflected light beams depends not only on the size of the reflecting surface that has become free, but also on its reflectivity bobbins with different degrees of reflection may lead to inaccurate results during the monitoring of the residual thread.
The use of bobbins with two chambers of different size for the purpose of the optoelectric recognition of a residual amount of thread has been known from U.S. Pat. No. 3,845,320 for a long time. All three bobbin flanges are made of a transparent material in one of the three disclosed exemplary embodiments, and the optoelectric recognition device is designed as a transmitted-light photoelectric cell. One of the bobbin flanges is designed as a reflecting surface in the other two exemplary embodiments, while at least one of the other bobbin flanges consists of a transparent material. The recognition device is designed as a reflected-light photoelectric cell in these two exemplary embodiments. There is a risk in all three variants that the reliability of the recognition device is compromised by insufficient transparency or reflectivity of the bobbin flanges as a consequence of contamination or damage to their surfaces.